Fluid applications are broad, varied, and complex, and each application has its own set of characteristics and requirements. Proper fluid selection and fluid blend development have a large impact on the success of the operation in which the fluid is used. For instance, in a typical industrial coatings operation, a blend of several fluids is used in order to get an appropriate evaporation profile. Such a blend must also provide the appropriate solvency properties, including formulation stability, viscosity, flow/leveling, and the like. The fluid blend choice also affects the properties of the dry film, such as gloss, adhesion, and the like. Moreover, these and other properties may further vary according to the application method (e.g., spray-on), whether the substrate is original equipment (OEM), refinished, etc., and the nature of the substrate coated.
Other operations involving the use of fluids and fluid blends include cleaning, printing, delivery of agricultural insecticides and pesticides, extraction processes, use in adhesives, sealants, cosmetics, and drilling muds, and countless others. The term "fluid" encompasses the traditional notion of a solvent, but the latter term no longer adequately describes the possible function of a fluid or blend in the countless possible operations. As used herein the term "fluid" includes material that may function as one or more of a carrier, a diluent, a surface tension modifier, dispersant, and the like, as well as a material functioning as a solvent, in the traditional sense of a liquid which solvates a substance (e.g., a solute).
The term "industrial solvent" applies to a class of liquid organic compounds used on a large scale to perform one or more of the numerous functions of a fluid in a variety of industries. Relatively few of the large number of known organic compounds that could be used as fluids find use as industrial solvents. Fluids that are used in large quantities have heretofore been selected because they can be produced economically and have attractive safety and use characteristics in manufacturing, consumer and commercial environments. Examples of important industrial solvents are toluene, the xylenes, and mineral spirits, n-butyl acetate, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIK), and butanol.
In addition to the problems with fluid and fluid blend selection mentioned at the outset, there is also the problem that, in most applications, at least some of the fluid evaporates and can escape into the environment. In some applications, such as in certain coating operations, it is intended that the fluid evaporate. This evaporative property causes environmental problems. Although many industrial coating operations, such as in original equipment manufacturing (OEM) and auto refinishing, utilize control equipment to capture &gt;95% solvent emissions, nevertheless at least some inevitably enters the atmosphere.
The United States Environmental Protection Agency (EPA) has developed National Ambient Air Quality Standards (NAAQS) for six pollutants: ozone, nitrogen oxides (NO.sub.x), lead, carbon monoxide, sulfur dioxide and particulates. Of all the NAAQS standards, ozone non-attainment has the greatest impact on solvent operations.
Solvents typically are volatile organic compounds (VOC), which are involved in complex photochemical atmospheric reactions, along with oxygen and nitrogen oxides (NO.sub.x) in the atmosphere under the influence of sunlight, to produce ozone. Ozone formation is a problem in the troposphere (low atmospheric or "ground-based"), particularly in an urban environment, since it leads to the phenomenon of smog. Since VOC emissions are a source of ozone formation, industrial operations and plants using solvents are heavily regulated to attain ozone compliance. As different regulations have been adopted, the various approaches to controlling pollution have evolved. Certain early regulations controlled solvent composition, while later regulations primarily concerned overall VOC reduction. A more recent regulation has combined VOC reduction with composition constraints. While the traditional source of emission reduction is large stationary industrial facilities, the EPA and other governmental entities have turned increasingly to consumer and commercial products for reduction in their solvent usage as an additional means to lower VOC emission and therefore ozone formation.
The EPA has developed a list of compounds with negligible photochemical reactivity, such as methane, ethane, acetone, and various halogenated compounds. The agency has determined that these compounds do not contribute appreciably to ozone formation, and granted them VOC exempt status. Numerous government and trade publications discuss VOC's, and information is readily available on the internet. See, for instance, http://www.paintcoatings.netVOCW97.html.
Various measurements of reactivity with respect to ozone formation are known. For instance, reactivity can be measured in environmental smog chambers, or they may be calculated using computer airshed models. See, for instance, Dr. William P. L. Carter, "Uncertainties and Research Needs in Quantifying VOC Reactivity for Stationary Source Emission Controls", presented at the California Air Resources Board (CARB) Consumer Products Reactivity Subgroup Meeting, Sacramento, Calif. (Oct. 17, 1995).
There has also been developed a "K.sup.OH scale", which provides a relative scale of the reactivity of VOC with the OH radicals involved in the complex reactions that produce ozone. See, for instance, Picquet et al., Int J. Chem. Kinet. 30, 839-847 (1998); Bilde et al., J. Phys. Chem. A 101, 3514-3525 (1997).
Numerous other reactivity scales are known and new reactivity scales are constantly being developed. Since this is a rapidly changing area of research, the most up-to-date information is often obtained via the internet. One example is Airsite, the Atmospheric Chemistry International Research Site for Information and Technology Exchange, sponsored by the University of North Carolina and the University of Leeds, at http://airsite.unc.edu.
Another way to measure the reactivity of a chemical in ozone formation is by using a technique developed by Dr. Carter (supra) at the Center for Environmental Research and Technology (CERT), University of California at Riverside. The CERT technique measures "incremental reactivities", the incremental amount of ozone that is produced when the chemical is added to an already polluted atmosphere.
Two experiments are conducted to measure the incremental reactivity. A base case experiment measures the ozone produced in an environmental smog chamber under atmospheric conditions designed to represent a polluted atmosphere. The second experiment called "the test case" adds the chemical to the "polluted" smog chamber to determine how much more ozone is produced by the newly added chemical. The results of these tests under certain conditions of VOC and nitrogen oxide ratios are then used in mechanistic models to determine the Maximum Incremental Reactivities (MIR), which is a measure of ozone formation by the compound.
The State of California has adopted a reactivity program for alternative fuels based on this technique and the EPA has exempted several compounds due to studies conducted by CERT. See, for instance, Federal Register 31,633 (Jun. 16, 1995) (acetone); 59 Federal Register 50,693 (Oct. 5, 1994) (methyl siloxanes), Federal Register 17,331 (Apr. 9, 1998) (methyl acetate). CARB and EPA have adopted a weight average MIR for regulatory purposes, wherein the weight average MIR of a solvent blend is calculated by summing the product of the weight percent of each solvent and its respective MIR value.
A list of compounds and their MIR values is available in the Preliminary Report to California Air Resources Board, Contract No. 95-308 William P. L. Carter, Aug. 6,1998. A table of known MIR values may be found on the internet at http://helium.ucr.edu/.about.carter/index.html. CERT obtains other incremental reactivities by varying the ratios of VOC and nitrogen oxides. A detailed explanation of the methods employed and the determination of incremental reactivities and MIR scale may be found in the literature. See, for instance, International Journal of Chemical Kinetics, 28, 497-530 (1996); Atmospheric Environment, 29, 2513-2527 (1995), and 29, 2499-2511 (1995); and Journal of the Air and Waste Management Association, 44, 881-899 (1994); Environ. Sci. Technol. 23, 864 (1989). Moreover, various computer programs to assist in calculating MIR values are available, such as the SAPRC97 model, at http:1/helium.ucr.edu/.about.carter/saprc97.htm.
Any of these aforementioned scales could be used for regulatory purposes, however the MIR scale has been found to correlate best to peak ozone formation in certain urban areas having high pollution, such as the Los Angeles basin. MIR values can be reported as the absolute MIR determined by the CERT method or as a relative MIR. One common relative MIR scale uses the Reactive Organic Gas (ROG) in the base case as a benchmark. The Absolute Reactivity ROG is 3.93 g O.sub.3 per gram ROG. This value is then the divisor for the absolute MIR of other VOCs, so each MIR is cited relative to ROG. All MIR values cited herein are relative to ROG=3.93.
Solvents currently viewed as essentially non-ozone producing are those which have reactivity rates in the range of ethane. Ethane has a measured reactivity based on the MIR method of 0.08. In fact, the EPA has granted a VOC exemption to certain solvents with reactivity values in this range including acetone (MIR=0.12) and methyl acetate (MIR=0.03).
However, the number of known materials having reactivities of 0.12 or less based on the MIR scale is relatively small. Moreover, it is a discovery of the present inventors that many if not most of the known fluids having acceptable reactivities with respect to ozone formation have other unfavorable performance characteristics, e.g., poor solvent properties, low flash point, inappropriate evaporative or volatility characteristics, unacceptable toxicity, unacceptable particulate matter formation, thermal or chemical instability (e.g., reactive to species other than NO.sub.x, and more particularly reactivity in solution), and as such has limited, if any, applicability in industry. For example, ethane, having an excellent MIR=0.08, is a gas under ambient conditions and hence is a poor choice as an industrial solvent. Methyl acetate has an excellent MIR=0.03 but a low flash point of about -12.degree. C.; acetone has an acceptable MIR=0.12 but is extremely flammable. As a further example, tertiary butyl acetate (t-butyl acetate) has an excellent MIR=0.04 but has limited thermal stability and is unstable to acid catalysts which may be present in an industrial operation.
Regarding particulate matter, the EPA has recently proposed standards for particulate matter under 2.5 microns in diameter ("PM2.5"). See 61 Federal Register 65638-65713 (Dec. 13, 1996). The proposal sets an annual limit, spatially averaged across designated air quality monitors, of 15 .mu.g/m.sup.3, and a 24-hour standard of 65 .mu.g/m.sup.3. Numerous discussions of this proposed standard are available on the internet, such as at http://www.cnie.org/nle/air.about.html, which cites numerous references (such as Wolf, "The Scientific Basis for a Particulate Matter Standard", Environmental Management (Oct., 26-31 1996)). As far as the present inventors are aware, the prior art has not addressed ways of meeting these proposed requirements, much less in meeting these requirements in conjunction with ozone reduction requirements.
Moreover, the present inventors have also discovered that in many applications, VOC exempt solvents cannot be used as a one-for-one replacement for conventional solvents. Rather the formulator must balance a number of performance factors to develop an acceptable solvent or solvent blend for a particular application. Some factors are more relevant than others for specific applications. Nevertheless, many performance factors are similar for a number of applications.
Numerous attempts have been made to utilize the concept of "environmentally friendly" fluids in practical applications. For instance, there are a number of cleaning and/or stripping formulations available that are said to overcome certain prior art environmental problems. Examples include a binary azeotrope of octamethyltrisiloxane with n-propoxypropanol (U.S. Pat. No. 5,516,450), hexamethyidisiloxane and azeotropes and other mixtures thereof (U.S. Pat. No. 5,773,403), a nonazeotropic mixture including a halocarbon and an oxygenated organic solvent component having at least 3 carbons, which may be, for instance, dimethylcarbonate (U.S. Pat. No. 5,552,080), and a composition comprising an amide and a dialkyl carbonate (U.S. Pat. No. 4,680,133).
In addition, there have been a number of patents and literature references to materials intended to replace chlorofluorocarbons (CFCs) as, for instance, blowing agents. These efforts address stratospheric ozone depletion, which is the opposite phenomenon addressed by the present invention. Examples include the use of dimethoxymethane and cyclopentane (U.S. Pat. Nos. 5,631,305; 5,665,788; and 5,723,509), cyclopentane (U.S. Pat. No. 5,578,652) and polyglycols (U.S. Pat. No. 5,698,144). Still further, a "non-ozone depleting" solvent comprising halogenated compounds and an aliphatic or aromatic hydrocarbon compound having 6-20 carbon atoms is disclosed in U.S. Pat. No. 5,749,956. Similarly, U.S. Pat. No. 5,004,480 describes a method for reducing the levels of air pollution resulting from the combustion of diesel fuel in engines comprising blending dimethyl carbonate (DMC) with diesel fuel and combusting the blended fuel in engines. U.S. Pat. No. 5,032,144 also discusses the addition of oxygenates, including dimethyl pivalate (methyl 1,1,1-trimethyl acetate) to gasoline (as octane boosters). The problems addressed by these patents do not relate to the problem of industrial solvent evaporation.
WO 98/42774 discloses a solvent-resin compositions which "do not contribute appreciably to the formation of ground based ozone". Organic solvents are selected based upon having "reaction rates with hydroxyl ion slower than ethane", and generally selected from halogenated solvents such as chlorobromomethane, methyl chloride, and the like. The only non-halogenated solvents that are suggested are n-alkanes (C.sub.12 -C.sub.18), methyl and t-butyl acetate, acetone, dimethoxymethane, and mineral oils.
However, heretofore there has been no general solution to the problem of ground-based ozone formation that also provides for a fluid with appropriate performance attributes for an industrial solvent.